1. Statement of the Technical Field
The inventive arrangements relate to wireless communication systems and more particularly to interference mitigation in wide-band communication signals.
2. Description of the Related Art
Wireless communication systems include receivers and transmitters that are arranged to facilitate communications using radio frequency signals. In a spread spectrum wireless communication system, pseudo random sequences (PRS) and/or Walsh modulation of a digital data signal can be used to spread the signal energy of a transmitted signal over a relatively wide bandwidth. Among other advantages, such spreading can reduce the negative impact of interference in the communication channel (as long as the interference is smaller than the processing gain of the spread signal). The improvement is most noteworthy in scenarios where the interfering signal is narrow-band in nature as compared to the signal of interest. The process of demodulating the spread spectrum signal inherently has the effect of spreading the energy of the narrow-band interfering signal over the full spread spectrum signal bandwidth, thereby reducing the effect of the interference. Further, various digital processing techniques have been developed which facilitate the excision or suppression of narrow-band interferers in spread spectrum signals. These methods include adaptive filter designs which operate in the time domain and in the frequency domain. In addition, spread systems can also be used to allow multiple users to share the same frequency (e.g. WCDMA and UMTS cellular systems).
In systems that perform filtering operations in the time domain, linear prediction methods are commonly used. Interference is predicted by an adaptive Finite Impulse Response (FIR) filter and is subtracted from the received signal. In systems that perform filtering operations in the frequency domain, the digital data samples are first converted from the time domain into the frequency domain via the well-known Fast Fourier Transform (FFT). Thereafter, narrow-band interference can more easily be recognized because the energy of the interfering signal is concentrated in a smaller bandwidth. Once the interference is mitigated, the digital data samples are converted back to the time domain via an Inverse FFT (IFFT). A subset of this method uses a time-varying FIR filter which adjusts its coefficients based on the estimate of the interferer's instantaneous frequency.
A conventional narrow-band excision system that operates in the frequency domain will commonly include several basic components. Such components can include an FFT based transformer, a power estimator, an excisor and an IFFT based inverse transformer. The transformer can include a windowing function to reduce the effects of spectral leakage, as is known in the art. The power estimator in such systems receives frequency domain data from the transformer and uses this information to determine a power vector. The excisor receives the frequency domain data from the transformer and is adapted to excise selected frequency bins from the frequency domain data. More particularly, the excisor selectively removes the narrow-band interference from the frequency domain data based on a determination of whether the power vector exceeds an excision threshold. After the frequency domain data has been processed by the excisor, it is communicated to the inverse transformer where it is converted back to the time domain. Note that narrow-band interference can cover a wide range of bandwidths up to ⅕ of the spread waveform bandwidth.